This invention is in the field of video recording and more particularly relates to a method for the precise positioning of a record/reproduce head on previously recorded material.
In the recording of video information on magnetic tape, relative motion is required between the magnetic record head and the magnetic tape, due to the bandwidth of video signals. In particular, not only is the magnetic tape moved past the magnetic recording head, but the magnetic recording head also moves relative to the tape.
The motion of the magnetic head relative to the magnetic tape can take on of several different forms. In the well known quadraplex format there are usually four magnetic recording heads positioned 90.degree. apart on a rotating drum. The drum rotates in a plane which is perpendicular to the surface of the magnetic tape with the axis of rotation parallel to the direction of motion of the tape. While the drum rotates a capstan physically moves the tape past the rotating drum. Consequently each head on the rotating drum records a signal in a segmented manner across the surface of the tape positioned perpendicular to the sides of the tape. In such a procedure, the video signal to be recorded is sequentially switched among the heads in such a manner to be supplied to the particular head which is in contact with the magnetic tape. In this manner a relatively large head to tape speed can be achieved which permits the recording of video information.
Audio information is also simultaneously recorded on the magnetic tape along with the video in a conventional manner employing a fixed positioned audio record head. Generally the audio tracks are physically positioned longitudinally above and below the segments of recorded video adjacent to the edges of the tape.
In the quadraplex format above described, a plurality of magnetic recorded segments are usually necessary to record a complete field of a television picture.
In yet another format used in the recording of video information on magnetic tape, the video information is recorded in segments positioned diagonally across the surface of the magnetic tape. This is accomplished by employing one or a plurality of magnetic recording heads which are positioned radially outward on the circumference of a disc which is rotated, hereinafter generally referred to as the scanner. The magnetic tape is routed around the circumference of the rotating plane in a helical fashion. Consequently as the surface of the magnetic tape is physically moved around the circumference of the rotating disc having one or a plurality of magnetic recording heads mounted thereon, the resulting signal which is recorded on the magnetic tape is positioned diagonally across the width of the magnetic tape. In this manner a relatively large head to tape speed can be achieved which permits the recording of video information.
Depending upon the particular format a complete television field is recorded in either a single diagonal magnetic track or a plurality of magnetic tracks.
Audio information is likewise simultaneously recorded on the magnetic tape along with the video in a conventional manner employing a fixed position audio record head. Generally the audio tracks are physically positioned longitudinally above and below the segments of recorded video adjacent to the edges of the tape.
For the purposes of clarity of discussion, the following discussion will be limited to the non-segmented helical format. Similar consideration as discussed below would apply to other recording formats, and would be apparent to those having ordinary skill in the art.
In the above described approaches which are typical of the manner in which video information is recorded on magnetic tape, correct positioning of the video information on the magnetic tape is absolutely essential to the successful playback of the information. Clearly video information which is incorrectly recorded would present problems in the subsequent playback, due to the complex practical considerations which are involved. This problem becomes even more acute if the interchange of previously recorded magnetic tapes is desirable between different video recording/reproducing machines.
The problems involved in the recording of video information on magnetic tape become even more demanding when it is desirable to place additional video information on a previously recorded magnetic tape in such a manner to match that previously recorded. This process, referred to as editing, involves the selected placement of additional record tracks adjacent to previously recorded tracks. The physical placement of the recorded tracks of the additional video information is very critical if satisfactory playback is to be achieved. In conjunction with the correct physical placement of the newly recorded tracks are similar critical requirements relating to the timing characteristics. This task involved basically two considerations: the physical placement of the magnetic track on the magnetic recording tape and the relative positioning with reference to time of the corresponding electrical signal on the magnetic track.
Referring first to the physical placement of the magnetic track on the magnetic recording tape, the magnetic track containing the new video information must be positioned with precisely the same distance and angular parameters as present on existing recorded magnetic tracks. In particular, the mechanical parameters of the angular placement of the new magnetic tracks, the physical length of the new magnetic track, the distance between the edge of the magnetic tape and the starting point of the new magnetic track and, in particular, the horizontal spacing between the new magnetic track and previously existing magnetic tracks must correspond.
From a practical standpoint, most of the physical parameters are either determined at the time of manufacture of the equipment, or are controlled by considerations not relevant to the present discussion. However, the single physical parameter which is relevant to the present discussion is the horizontal or longitudinal spacing between the new magnetic tracks and those previously existing on the magnetic tape. Due to the nature of the process of placement of the magnetic tracks on the magnetic tape, the distance between tracks will generally be regular, again being determined by considerations not relevant to the present discussion. However, the significant parameter of relevance is the spacing between the last magnetic track of previously existing information on the magnetic tape and the new track to be added. This distance is critical, and must be precise, because once set it will determine the accuracy of the reproduction of subsequent tracks.
There is generally recorded on one of the above described audio tracks on magnetic tape, along with video information, a series of regularly spaced pulses. This track is known in the art as the control track. There is also affixed to the above discussed scanner a tachometer which produces a pulse indicative of a particular angular position on the scanner, hereinafter referrred to as the tach pulse. As the control track pulses directly relate to linear tape position, and the tach pulse directly relates to angular position of the scanner, the relative phase difference between the two is directly proportional to the horizontal or longitudinal spacing between sequential magnetic tracks. Consequently varying this phase angle results in a directly proportionally variation in the horizontal or longitudinal spacing of the magnetic tracks.
In particular, in the recording of video information, by varying this phase angle, the corresponding distance betwen the magnetic tracks which are recorded on the magnetic tape will be varied, e.g., increasing this phase angle increases the horizontal spacing between the last previous track and the first new track.
In a similar fashion, in the reproduction of video information from magnetic tape, varying this phase angle will directly control the alignment of the reproduce magnetic head with the magnetic tracks previously recorded on the tape.
Consequently, by varying the phase angle, the horizontal distance between sequential magnetic tracks can be directly controlled.
In the process of reproducing previously recorded material on magnetic tape, it is frequently necessary to adjust the above described phase angle to effect correct alignment between the reproduce magnetic head and the video information recorded on magnetic tape. As the signal from the reproduce magnetic head will be maximum when the reproduce magnetic head is in direct alignment with the previously recorded magnetic tracks, by monitoring the level from the reproduce magnetic head as the above discussed phase angle is varied, correct alignment can be achieved. This process is referred to in the art as adjusting the "tracking".
In a similar fashion when it is desired to perform an edit operation, i.e., record new video information adjacent to previously recorded video information, by performing the above described tracking adjustment, correct horizontal placement of the subsequently recorded new video information will result.
Consequently prior to performing an edit operation, it is essential to adjust tracking, i.e., adjust the phase angle between control track pulses and the pulse produced by the scanner tach to achieve a maximum signal level from the reproduce magnetic head. Performing this adjustment will ensure that the horizontal spacing between the existing magnetic tracks and the subsequently recorded magnetic tracks will be precise.
In the past, this has been accomplished by the operator manually adjusting a potentiometer which controls the above described phase angle for a maximum level indication on a meter which monitors the level from the reproduce magnetic head. This procedure has a number of disadvantages. First, as the operation requires a human operator, the results are subject to human error, including the complete omission of the procedures. Furthermore, the resulting accuracy of horizontal placement of the video information on magnetic tape is directly dependent on the accuracy of the tracking adjustment. Clearly a precision adjust is highly desirable.
In addition to the adjustment of the horizontal spacing of the subsequently recorded magnetic tracks with reference to previously recorded magnetic tracks, the placement of the new video material with respect to time on the magnetic track is likewise critical.
A video signal has a well defined electrical format with respect to time which provides for the placement of video information in particular time segments thereof. A video signal also contains a number of well defined periodically occurring pulses which serve as reference points with respect to the location of video information within the video signal itself. These reference points are useful in the processing of the information contained in the video signal. One such reference pulse is referred to as the vertical pulse, and serves to identify the end of video information which defines a television field.
The vertical pulse is particularly important in the recording of video signals, as it is used to control the physical placement or positioning of video information with respect to time on a magnetic track.
For the purposes of illustration, the following discussion will assume that the video information for a complete television field is recorded on each magnetic track diagonally positioned across the width of the magnetic tape, as above discussed.
As each magnetic track is recorded in a serial fashion, it is clear that linear position along a particular magnetic track corresponds directly to time. Consequently in the recording of a video signal, the various portions thereof each will be recorded or placed in a particular physical location along the magnetic track which directly corresponds to the time at which the various portions occurred. In particular, as the vertical pulse occurs at the end of each field of video information, and linear position along the magnetic track on which the signal is recorded corresponds directly to time, the vertical pulse will be recorded at a particular position on the magnetic track toward the end. Furthermore, as the occurrence of the vertical pulse is regular in time, the corresponding linear position of the vertical pulse recorded on tape will be regular, i.e., the vertical pulse will be placed in the same relative linear position along the magnetic tracks for each magnetic track recorded.
When it is subsequently desired to record additional video information on a magnetic tape on which video information has been previously recorded, it is highly desirable for the resulting magnetic tracks of the new video information to have the identical physical parameters of the previously recorded video information.
In particular, it is not only desirable that the horizontal or longitudinal spacing between each magnetic track remain constant, but also that the video information placed on the magnetic track be positioned in an identical manner to that which was previously recorded. As the location of the vertical pulse is the same for all video signals, clearly the desired result can be obtained by placing the new video information on the magnetic track in such a manner that the vertical pulses associated therewith be in the same physical linear position on the magnetic tape as the vertical pulses associated with previously recorded video information.
In the past, when it has been desired to perform an edit operation, i.e., record additional video information on a magnetic tape containing previously recorded video information, the recording apparatus achieves the alignment of the vertical pulse on the newly recorded information with that previously recorded on the magnetic tape in the following manner.
Prior to the recording of the additional video information, the recording apparatus determines the corresponding physical location of the vertical pulse on the existing magnetic tracks. Thereafter, upon the recording of the new video information, the recording apparatus positions the vertical pulses associated with each field in the corresponding linear position on each magnetic track recorded in the same position as that determined from the previously recorded material. This can be done by adjusting the phase angle between the reference pulses and the scanner tach pulse. In theory, the resulting newly recorded video information will be placed on the magnetic tape in the same relative physical location along the magnetic tracks as the previously recorded information, i.e., the corresponding linear position along the respective magnetic tracks of the vertical pulse associated with each of the previously recorded magnetic tracks of video information will be in the same relative location along the respective magnetic tracks for the newly recorded video information.
In practice, however, this is not the case, due to the error associated with the measurement process. In particular, the associated electronic circuitry, and mechanical system involved with the positioning of the newly recorded video information on the magnetic track in such a manner as to result in the placement of the vertical pulse in the same linear location as vertical pulses associated with previously recorded video information will have an error associated with the positioning process. In particular, the linear positioning of the vertical pulse along the newly recorded magnetic track will be in relatively close agreement with the linear position of vertical pulses associated with previously recorded video material, differing by a finite amount which is the error associated with the inability of the electronic circuitry to effect exact alignment. In practice the amount of this error can be quite small; however, it is present in a finite amount.
The associated error, notwithstanding the fact that it may be quite small, is significant due to the method employed in the alignment of vertical pulses. In particular, each time an edit function is performed, i.e., additional video information is to be recorded on magnetic tape, the position of the vertical pulse on the immediately preceeding previously recorded video signal is used as the reference position against which the vertical pulse of the new video information will be aligned. Consequently, the amount of error associated with the ability of the electronic circuitry to properly position corresponding vertical pulses, albeit small, is nevertheless finite and additive. Consequently when a series of edits are performed on a magnetic tape, the amount of error present in the placement of the vertical pulse is additive and cumulative. While the amount of error may in itself be quite small, it can still produce undesirable effects due to the additional nature of the process when numerous sequential edits are performed.